Measurement of distances between sensor modules under water and a reference point, which, for example, is a vessel towing the sensor modules behind it, has been a well-known problem area which up to now has been solved by measuring the length of the line or wire between vessel and sensor modules. An alternative may be to send an acoustic signal from a reference point to a sensor module, and then measure the time it takes to receive a responding signal from the sensor module. This measuring alone will however make an uncertain measurement.
Accurate measurement of the line length has been found to be difficult in practice since the line may stretch, curve or twist. This is the case whether the length of line is measured manually as it leaves the vessel, or whether the number of rotations a winch has made in paying out and adjusting a certain length of line is measured. Furthermore, the last-mentioned is dependent on how the line is coiled, which may be different from one time to another. The problem is exacerbated when the distance between the sensor modules and the vessel becomes large, i.e., several hundred metres. A line or wire will then be able stretch quite considerably and become longer than when it is coiled up
Today there are different devices for measuring distances under water using a measuring principle comprising transmission and reception of sound waves. This comprises primarily the use of sonar or echo sounders.
The principle of these devices is to emit a sound wave and measure the time it takes before the same sound wave is reflected. To find the distance from, for example, a vessel to one or more sensor modules, the sonar principle can be used by emitting a sound wave from the vessel and receiving reflections or transmitted signals from sensor modules towed behind the vessel. The time the sound waves take to pass to or from the sensor modules will then be proportional to the distance between them and the vessel. The last-mentioned principle is also used to find the distance between two or more sensor modules under water.
WO-2010/090526 A1 describes a method for determining difference in distance from a reference point to at least two sensor modules where the reference point and the sensor modules are under water. The sensor modules comprises means for sending and receiving acoustic signals and the reference point comprises a hydrophone for receiving acoustic signals from the sensor modules. Said method describes the difference in distance and not the distance itself.
U.S. Pat. No. 5,214,617A shows an example of a standard technique and describes a system and a method for determining the distance between individual hydro acoustic transceivers placed on objects under water. The system uses synchronization of clocks in sensor modules comprised in the system.
It is desirable with a more accurate distance determination where several measured parameters are coordinated for determining distance.
Calculation of distance from one point to another can be performed in two ways. The first is to measure travel time of a signal by using clocks suck that the receiver of the sent signal knows when it was sent. It is the only necessary to send the signal only one way, e.g. from a sensor module to a reference point having a hydrophone for picking up the signals.
The other known method is to send a signal from one sensor to another and measure the time before the signal is returned.
By sending signals from two or more sensor modules with a known time difference, the signals that are received by a hydrophone will indicate difference in distance between the sensor modules. This may be useful information per se if, for example, it is desirable that two or more sensor modules at all times should have a constant difference in distance or the same distance to, for example, a vessel towing them behind it.
A well-known problem when using sound waves under water is furthermore that the sound propagation velocity is dependent on a number of factors such as water temperature, salt content, pressure, etc. These will vary according to location, season, current conditions etc. It is the water temperature that has the greatest impact for the measuring result.
Several suppliers of sensor modules for use under water use a fixed velocity of 1500 m/s for propagation of sound waves, or the velocity of sound in water “is set” manually by look up relevant values for the sound velocity in water at a given temperature.
Water temperature in the vertical direction changes with depth. Warm water rises and cold water sinks down. In addition, the sun will warm the surface water.
The water temperature in the horizontal direction may also vary depending on distances and where measurements are made. For example, water close to the shore will have a higher temperature than water that is further from the shore.
There are some devices which have a temperature sensor incorporated for measuring the temperature locally in the water where the device is located. The disadvantage of such systems is that a water temperature measured locally at the location of, for example, an echo sounder may be very different from the water temperature along the whole propagation path of the sound wave. This will be the case, in particular, across large distances. The sound velocity that is then used in calculations of distances under water will be incorrect and result in large deviations in calculated distance in relation to the real distance.
The present invention solves the last-mentioned problem by measuring temperatures at at least one or more points along the path the sound will propagate so as to collate them for a more accurate calculation of distances under water. This is done by transferring real measured temperature or calculated sound velocity from measured temperature to a unit connected to a hydrophone at the reference point for calculating distance under water. Greater accuracies will thus be obtained by a measurement of this kind.
Combining received signals regarding time of arrival and time of flight the emission of sound waves from at least two sensor modules and simultaneously adjusting the sound velocity in relation to measured water temperature will allow difference in distance to the sensor modules to be found with greater accuracy. This may, for example, be used for precise adjustment and positioning of a trawl.